(a) Field of the Invention
The present invention relates to a new type of distribution transformer in which the electric circuit is constituted by two or more sub-assemblies in the form of double pancakes which are juxtaposed and molded together in an insulating material whereby to form a rigid insulating frame which is cooled internally by heat exchange tubes in which circulates a cooling fluid, and wherein on one or more of the legs of the frame there is wound a magnetic circuit which is formed by one or more toroidal coils made from ribbon of ferro-magnetic material with the magnetic circuit also being cooled by the cooling fluid that circulates inside the frame.
(b) Description of Prior Art
Conventional distribution transformers are used to step down the electrical voltage at various user locations on a power distribution line. The conventional power transformer, as presently known, consists essentially of a magnetic circuit having a metal core formed from a plurality of thin superimposed plates disposed parallel and connected to each other. Nowadays, almost exclusively, these plates are crystal-oriented and hence have a strong anisotropic structure. Further, these plates exhibit a high performance in their laminated direction but have magnetic characteristics that are very mediocre in the direction transverse to the lamination. The primary and secondary windings of such distribution transformers are metallic conductors, for example insulated copper wires having a circular or rectangular cross-section, wound in a bobbin about one or more of the legs of the magnetic core or frame. The electrical voltage applied to the primary winding of these distribution transformers is of the order of several kilovolts but can go as high as tens of kilovolts, whereas the voltage appearing at the terminal ends of the secondary winding is of the order of a few hundred volts.
The range of known distribution transformers extends from a few kVA to about 300 kVA. The magnetic and electric circuits are immersed in a mineral oil which is contained in a metal transformer housing. This oil serves as an insulator and also participates in the cooling of the transformer. The major disadvantages of such conventional distribution transformers are set forth below.
1. Prior art distribution transformers being immersed in an oil that is inflammable are vulnerable to fire or explosion in the event of defects or over-heating of the transformer, and furthermore, the weight of such oil represents approximately 25% of the total weight of the transformer, excluding the housing and the terminals.
2. The concept of prior art distribution transformer is such that it is impossible to eliminate "hot points" which accelerate the aging of the oil and all of the electrical insulation in the transformer construction.
3. The concept of prior art distribution transformers is such that it is impossible to completely utilize the anisotropy of the crystal-oriented plates forming the magnetic circuit.
4. The oil-air heat exchangers on the transformer housing for the cooling of the oil inside the housing are not very effective.
5. Prior art distribution transformers also are characterized by energy losses arising from hysteresis and Foucault currents, these losses being continuous and relatively large.